3 blade on a cruiser with a lot of horsepower

[farmo83]

I've seen on a few threads and in Dave's write up on the 24 Schiada wood deck about how dangerous and unsafe it is for V drive cruisers with big power to run 3 blades. Why is this?

 

[Racey]

Its not dangerous per se, the boat will just have a tendency to ride up on the nose more and chine walk as more blades almost always equates to more transom lift. You just won't be able to run above 85-90 if its a Schiada style boat. Running stainless props with big HP is dangerous, 2 or 3 blades, they have a tendency to crack and throw blades with big torque.

 

[AzGeo]

I feel that the 'shape, diameter and most importantly the blade rake' are the major factors when testing a 3 + blade prop. Before you put any prop on this ultra high performance boat the prop needs to be either a 'C N C billet' and/or a welded unit that has been 'X-rayed' and stamped "certified", prior to use. Blade rake reduces 'tail lift' and certain kinds of 'special 90 degree cupping will also greatly reduce 'tail lift', so knowing what you want makes this entire process go much smoother. I like the quickness of a 3 blade, and remember, the entire 'power load' is being transmitted through two welds, from the hub to the blades. A three blade divides that load by aprox 1/3 at the welds, so it should last a little longer under very heavy stresses. Old time inboard props were built with 'little blade rake' which means that the 'blade tips' were almost directly over the prop hub, this allowed a large amount of water to 'fly off the ends of each blade' at higher speeds. Read that high prop RPMS. As blade designs progressed, builders found that 'laying the blades back behind the hub' (more blade rake) caused a smaller diameter measurement but kept the bite surface area to move the boat. A few prop companies use a '90 degree cup that may progress all the way around the blade tip end, and this too reduces tail lift. Two blades also have a 'zone of vibration' which are difficult to 'tune out' when building the props, some brands have reduced it, some have compromised the overall performance just to reduce the vibration itself. I see that within the past 15 years, props have created better overall top speeds, better control, longer life, and it's all due to those boaters and prop makers who take the time to test all known variables and not just 'sell off the shelf'.

 

[500bbc]

????
1400HP, 110 in the 1/4, 21' Howard

Racey and George are spot on on the crack check though.

 

[vdcruiser]

Had that happen a couple of times on my 21' Howard with a stainless prop, what did the shaft look like and what other collateral damage was there. I use only steel now and the 2 blade is 5 or 6 mph faster than the 3. I would like to know more about the CNC props.

 

[SFV2RVR]

Had that happen a couple of times on my 21' Howard with a stainless prop, what did the shaft look like and what other collateral damage was there. I use only steel now and the 2 blade is 5 or 6 mph faster than the 3. I would like to know more about the CNC props.

Ron and Joey Grose make the best forged props out there! If you have a prop that works well already, they can digitize it for you and make the exact one.

 

[500bbc]

Had that happen a couple of times on my 21' Howard with a stainless prop, what did the shaft look like and what other collateral damage was there. I use only steel now and the 2 blade is 5 or 6 mph faster than the 3. I would like to know more about the CNC props.

Broke the tranny in half and scattered the flexplate.
V-drive was OK, replaced the shaft to be sure.

Nice note on the scattered plate, he had Ron at BA in Parker run the boat up to Echo while we were there after it went back together.
I leaned in the boat and there was gas everywhere. Piece of the plate about the size of eraser went through one side tank that was carpeted over.:yikes

 

[ctchndltgo200]

????
1400HP, 110 in the 1/4, 21' Howard

Racey and George are spot on on the crack check though.

I am new to the boating community and especially V-drive Flatties. But I have metallurgical experience, and yes, you not only have centrifugal forces to contend with, you also compound this affect by adding the rotational forces on each individual blade. As the blades flex, can be no more than thousandths of an inch, this crystallizes (stretches and re-compresses the molecular structure) the flex point weakening it, allowing the stress point to stretch and eventually part. I hope my thoughts help some.:blah::blah::blah: I know

 

[Rivrat]

http://www.riverdavesplace.com/forums/showthread.php?59824-3-Blade-Prop-and-Big-HP

Check thread